1. Field of the Invention
The present invention relates to an optical transmission system for transmitting wavelength division multiplexed (WDM) signals. More specifically, the present invention relates to an optical transmission system having optical repeaters arranged along a transmission line, where each optical repeater has a gain equalizer that can be selectively enabled or disabled.
The present invention also relates to an optical transmission system having an add/drop apparatus with a plurality of individually selectable filters for adding/dropping individual wavelength signals from a WDM signal.
2. Description of the Related Art
Conventional optical transmission systems transmit optical signals along an optical fiber transmission line. Erbium-doped fiber amplifiers are typically arranged along the transmission line to amplify the optical signals.
FIG. 1 is a graph illustrating the wavelength versus power output of a typical erbium-doped fiber amplifier (EDFA). Referring now to FIG. 1, an EDFA has gain peaks at wavelengths of approximately 1533 nm and 1558.5 nm, due to the emission and absorption characteristics of erbium. Therefore, in underwater, or submarine, optical transmission systems, an EDFA is typically used for the transmission of signals with a wavelength of 1558.5 nm.
FIG. 2 is a graph illustrating the gain versus wavelength relationship of an EDFA near the gain peak of 1558.8 nm. Referring now to FIG. 2, an EDFA has a single-peak characteristic curve with a bell shape centered around a gain peak at the wavelength of 1558.8 nm.
Referring again to FIG. 1, the curve is relatively flat over a specific wavelength range. As a result, wavelengths in this range will experience relatively equal gain when a single EDFA is used. However, when optical signals are transmitted over long distances (for example, approximately 5000 km) and thereby travel through many EDFAs or repeaters (for example, approximately 70 EDFAs or repeaters), wavelength-gain relationships of each EDFA are accumulated due to the self-filtering effect of an EDFA. As a result, the total characteristic curve is represented by a steep curve.
For example, FIG. 3 is a graph illustrating the wavelength versus power level caused by the cumulative characteristics of a plurality of EDFAS. Referring now to FIG. 3, the curve is relatively steep, and can be contrasted to the relatively flat curve in FIG. 1.
The step curve in FIG. 3 causes many problems in transmission systems using wavelength division multiplexing (WDM), since different wavelengths experience a significantly different effect.
For example, FIG. 4 is a diagram illustrating the wavelength versus power level caused by the cumulative characteristics of a plurality of EDFAs, for a WDM signal having eight wavelength signals multiplexed together. Referring now to FIG. 4, the optical signal-to-noise ratio (SNR) of the WDM signal greatly deteriorates because of the self-filtering effect for wavelengths .lambda.1 and .lambda.8, which are far from the wavelength having the peak gain. This deterioration is so serious that signal transmission is virtually impossible.